Collimators are optical devices, belonging to a family of devices known as collimators or telescopes, which include products called beam expanders and condensers. Such devices, advantageous to the laser-equipped machine tool industry, increase or decrease the size of and change the divergence or angular characteristics of a light beam passed through it. Such devices also have other characteristics and functions known to those skilled in the trade. Manufacturers of laser optics publish literature providing information on design variations and examples of use, for example: II-IV Incorporated publication 1685 Revised 3/92, Beam Expander-Condensers, Copyright 1988.
Collimators may be constructed of transmissive optics such that the light beam is passed through the optics. They are commonly used in laser equipped machine tools up to the approximately three kilowatt power level and sometimes above. Use of transmissive collimators with lasers with power levels above three kilowatts becomes increasingly problematic due to limits on energy density that transmissive optic materials can withstand and due to a phenomenon called thermal lensing. Thermal lensing is the distortion of an optical component caused by heat absorption typically from an input beam of light. The distortion can influence the divergence and mode quality of the beam passing through or reflecting from the optical component and cause detrimental shifts of focus position and diameter.
Collimators are also constructed of reflective optics, combinations of flat and shaped mirrors, such that the light beam is reflected from these optical elements. Reflective optical materials can withstand greater energy densities without damage and thermal lensing is not as severe in reflective optics as compared to transmissive optics. Thus reflective collimators are more suitably used in high power laser applications.
Known reflective collimators consist of four reflective optics. By adjusting the distance between specific optics the divergence of the output beam can be varied. U.S. Pat. No. 5,442,436 illustrates such a system as does II-VI Incorporated sales literature "Model RBC-Dx Reflective Beam Collimator" dated September 1992. The mirrors are arranged in pairs in parallel planes. To change the size of the output beam one pair of mirrors is moved parallel to the input and output beams and the distance between the pairs is changed.
It is advantageous to reduce the number of optical components used in the beam delivery system. Each optic adds complexity to the task of aligning a beam path, is a possible source of beam distortion and absorbs some energy from the beam reducing overall system efficiency. Optical elements degrade with use. Each optical element adds to the overall cost of system maintenance.